The Prime Minister was challenged over Conservative plans to cut welfare, while Ed Miliband said there would be no Labour government if it meant relying on the support of the SNP.

In a 90-minute Question Time special, the two leaders and Nick Clegg faced a studio audience separately in Leeds Town Hall, in a format devised after Mr Cameron refused to take part in a head-to-head encounter with his Labour rival.

The Prime Minister rejected suggestions that he was planning to slash £8billion from child benefit and child tax credit, but both Mr Miliband and Mr Clegg replied that his comments did not amount to a guarantee that the benefits were safe.

Mr Miliband said he did not accept that the last Labour government overspent and promised the party was "absolutely, deadly serious" about balancing the books in the next parliament.

The Labour leader also gave the strongest indication yet of his unwillingness to strike a deal of any kind with the SNP in the event of a hung parliament, telling the audience: "If the price of a Labour government is a coalition or a deal with the SNP, it is not going to happen."